Memory B cells are key contributors to the indiction and maintenance of humoral immunity. Characterization of the memory B cells response to SIV will be important in an vaccine strategy to elicit a neutralizing antibody response. In this research application we propose to develop reagents to specifically identify macaque memory B cell populations. In proof of principle studies we have produced monoclonal lamprey VLR antibodies that are specific for memory B cells in humans. We propose to develop novel monoclonal VLR antibodies to macaque memory B cells. Unlike mammalian antibodies, the VLR antibodies of the jawless sea lampreys utilize leucine-rich repeats as basic structural units. The radically different protein architecture combined with the large evolutionary distance ofthe sea lamprey allows VLR antibodies to be made that recognize antigenic structures which are not detetcted by mammalian immunoglobulin-based antibodies for structural or tolerogenic reasons. To generate monoclonal VLR antibodies, we will immunize sea lamprey larvae with purified primary B cells or B cell lines. VLR antibody expression libraries will be screened by flow cytometry and monoclonal VLR antibodies that specifically interact with memory B cells or subpopulations thereof will be used as affinity reagents to identify the antigen recognized by the VLR antibody. The VLR antibodies from this core will be provided to research groups to analyze the memory B cell responses to vaccination or SIV challenge. In a complementary approach, we will generate conventional mammalian monoclonal antibodies to FCRL4, an immunoreceptor that In humans is expressed by a subpopulation of mucosa associated memory B cells with enrichment in antibody specificity for the HIV gp120 protein. A fusion protein consisting ofthe extracellular domain of FCRL4 fused to the Fc-domain of IgG will serve as the immunogen and the specific monoclonal antibodies will be identified by flow cytometry. The long term goal of these studies is to generate novel reseach reagents that will facilitate the analysis of memory B cells responding to SIV challenge or vaccination.